Modern commercial aircraft are typically designed to fly at high subsonic cruise Mach numbers to reduce the time spent aloft while avoiding the system complexities associated with supersonic flight. Accordingly, the aerodynamic performance of advanced subsonic aircraft wings is typically optimized at Mach numbers of from about 0.80 to about 0.85. However, the aircraft must demonstrate controllable flight at higher Mach numbers, which the aircraft may experience during high speed bursts and/or dives. At these conditions, the airflow passing adjacent to the wing (and particularly, the airflow passing adjacent to the upper surface of the wing) may accelerate to supersonic velocities before passing through a normal shock and decelerating to subsonic velocities upstream of the wing trailing edge.
One problem associated with the occurrence of adjacent supersonic and subsonic regions of the flow on the wing upper surface is that the static pressure in subsonic regions can be significantly higher than the static pressure in supersonic regions. Accordingly, the wing may be subject to atypical pressure gradients, which can stretch the capability of the control system. A further drawback is that the position of the shock wave separating supersonic flow from subsonic flow can shift significantly over the chord length of the wing, depending on factors including the aircraft angle of attack, freestream Mach number, sideslip angle and control surface deflection. The position of these shock waves is not always advantageous, and can limit design options to meet certification requirements.
One approach for addressing the foregoing drawbacks is to install a “shock bump” on the wing upper surface. The shock bump can include a generally smooth, faired projection that reduces the Mach number at which the shock wave occurs (thereby reducing shock losses), and can also provide a single location at which the shock tends to anchor, even when the freestream Mach number and/or angle of attack change. While the shock bump can improve the controllability of the airfoil at near sonic flight conditions, the level of increased stability and control that it provides may not be sufficient for all wing designs and/or may not be optimal, particularly for airfoils designed for cruise at Mach numbers close to 1.0.